1. Field of the Invention
This invention relates to a hydroformylation process. More particularly, it relates to an improved and advantageous process for producing aldehydes by hydroformylation of a lower olefin in an organic solvent in the presence of a rhodium complex and a trisubstituted phosphine, which comprises adding to the reaction system a diphosphinoalkane of the general formula ##STR2## in an amount of 0.20 to 2.5 equivalents per gram atom of the rhodium present in said rhodium complex, wherein, in the formula, A.sup.1 and A.sup.2 are each aryl, R.sup.1 and R.sup.2 are each aryl or saturated hydrocarbon residues containing one or more carbon atoms, and Z is an alkylene radical whose principal chain contains 2 to 5 carbon atoms which optionally may be further substituted with one or more lower alkyls, whereby the catalyst life is markedly prolonged.
2. Description of the Prior Art
The hydroformylation reaction comprising reacting a lower olefin, typically ethylene, propylene or butene, with a hydrogencarbon monoxide mixture in an organic solvent in the presence of a rhodium complex and a trisubstituted phosphine is well known and is being utilized, for instance, in the commercial production of butyraldehyde from propylene.
Rhodium complexes as hydroformylation catalysts have several advantages over cobalt catalysts. For example, the rhodium complexes can effect the reaction under much milder conditions (lower temperature, lower pressure) and bring higher selectivity toward normal aldehydes, and therefore are more suited for use in commercial production. However, the rhodium complexes are very expensive. Therefore, from an economic point of view, the value of said complexes as catalysts in commercial hydroformylation processes is very dependent on the life thereof. A number of attempts have so far been made to maintain the catalytic activity of the rhodium catalysts under the hydroformylation conditions for a prolonged period of time, leading to various proposals, which can be classified roughly into the following three categories:
(1) Inhibition of thermal degradation of the rhodium complexes under the reaction conditions and of formation of inactive, highly-carbonylated rhodium complexes by carrying out the reaction while keeping such reaction conditions as rhodium catalyst concentration, trisubstituted phosphine concentration, carbon monoxide partial pressure and reaction temperature each within a very limited or narrow range [see, for example, German patent application No. (abbreviated as DTOS) 2,715,685];
(2) Hydroformylation in the presence of a trace amount of oxygen in the reaction system (see, for example, DTOS No. 2,730,527); and
(3) Hydroformylation while controlling the concentration in the reaction system of high boiling byproducts, which can serve as catalyst poisons below a specific level (see, for example, British Pat. No. 1,338,237 and DTOS No. 2,721,792).
There is, however, still room for improvement in practicing these processes. Thus, in case the measure described in (1) above is taken, the lowered reaction temperature and the increased trisubstituted phosphine concentration cause reduction of the reaction velocity, whereby it is required to use the expensive rhodium catalyst at a higher concentration so as to compensate for said reduction; accordingly measure (1) is considered disadvantageous from an economic point of view. Regarding the method (2) above, the trisubstituted phosphines and the product aldehydes are not stable against oxygen but may be converted into trisubstituted phosphine oxides and organic carboxylic acids, respectively, and as a result not only the catalytic is reduced, but also the product aldehydes are subjected to undesirable secondary reactions. The controlling of the concentration of high boiling byproducts below a certain level according to the method (3) above includes in industrial practice, frequent regeneration, activation and recovery of the rhodium catalyst which are accompanied by losses of the rhodium catalyst and the tri-substituted phosphine. Even when such measures as mentioned above are taken, decrease in the catalytic activity is still frequent during the reaction, making it unavoidable to regenerate, activate and recover the rhodium catalyst repeatedly. The procedure is complicated thereby to that extent, and losses of the rhodium catalyst and the trisubstituted phosphine result especially in the regeneration step. Because of these and other problems, the known methods of maintaining the activity of the rhodium catalyst remain to be further improved.